The invention relates to a threaded connection and more particularly to a threaded connection for a rod assembly made of rod sections of a trench-work apparatus.
The term “trench-work apparatus” within the context of the invention refers to apparatuses configured to transmit drive power from a drive apparatus via a rod assembly to a tool arranged on the rod assembly. These include in particular ground-boring machines used to introduce ground boreholes and in particular horizontal boreholes in the ground. Typically, the drive apparatus transmits pushing or pressing forces and a drive torque via the rod assembly of the ground-boring machine to the tool embodied as a drill head. However, trench-work apparatuses exist where only pushing or pulling forces are transmitted via the rod assembly to a corresponding tool. These include in particular trench-work apparatuses used to widen or pull out existing boreholes in the ground or previously installed pipes and to insert at the same time, if necessary, a new pipe. The trench-work apparatuses are frequently constructed so that they can be used for producing ground boreholes as well as for pulling operations, i.e., for further widening an existing borehole or an old pipe and for pulling in a new pipe, respectively. A pilot borehole can thus be introduced initially with the same trench-work apparatus, wherein a pilot bore head is advanced through the ground by pushing, until the pilot bore head reaches a destination shaft, whereafter the pilot borehole is widened by replacing the pilot bore head in the destination shaft with a widening head and the drilling assembly is retracted. Optionally, at the same time the widening head is pulled in, a new pipe connected to the widening head can be pulled into the widened ground borehole.
The rod assembly of such trench-work apparatuses usually includes several rod sections, which are successively connected with each other commensurate with the advance of the borehole. The individual rod sections are connected via coupling elements, wherein in addition to the plug connections described in DE 196 08 980, in particular, threaded connections are widely used. Important advantages of threaded connections are their low costs and the ability to screw them together automatically in a simple manner. However, a substantial disadvantage of threaded connection is that they frequently represent the weakest points of the rod assembly, caused by the relatively small diameter in the region of the threaded plug of the threaded connection as well as the geometry-related high stress concentration fatigue of the thread itself.
Conventional types of threads used for rod assemblies of trench-work apparatuses are API threads as well as round threads according to DIN 20 400. However, the service life of such thread shapes has proven to be too short in daily operations of the trench-work apparatuses.
Therefore, alternative thread shapes have been developed which are particularly constructed for the specific loads of the rod assembly that occur in trench-work apparatuses. DE 198 03 304 A1 discloses a threaded connection for connecting the rod sections of a rod assembly, wherein the rod assembly is to be used, in particular, for rotary percussion drilling. Important characteristics of these threaded connections are the asymmetric form of the thread with different pitch angles of the flanks that receive the load as well as the flanks that do not receive the load in rotary percussion drilling operations and the provision of an unthreaded insertion section. The flanks that do not receive the load should also be formed as a segment of the circle. The threaded connection disclosed in DE 198 03 304 A1 has turned out not to be sufficiently robust in day-to day operations.
EP 0 324 442 B1 and the U.S. Pat. No. 5,060,740 also disclose asymmetric threaded connections designed for use in connecting rod sections of ground-drilling machines. The threaded connections disclosed in these documents are characterized in that the root of the thread forms a segment of an ellipse. This should facilitate a transition from the root of the thread to the planar flanks of the thread without stress concentration.
WO 2006/092649 A1 discloses threaded connections for rod assemblies of ground-drilling apparatuses where the root of the thread forms a segment of an ellipse. The thread shapes disclosed in this document can be asymmetric as well as symmetric.
None of the conventional thread shapes have so far been able to distinguish themselves by an adequate service life when used for connecting rod sections of a rod assembly of a trench-work apparatus.
Based on the state-of-the-art, it was therefore an object of the invention to provide a threaded connection, in particular, with improved service life, which has an increased service life in particular in an application for connecting rod sections of a rod assembly of a trench-work apparatus.